1. Field of the Invention
The present invention relates to light sources that are used for optical communications. More particularly, the present invention relates to a multi-wavelength light source which can output multi-wavelength light in a multiplex system that includes a plurality of channels having differing wavelengths.
2. Description of the Related Art
In wavelength division multiplexed optical communication systems, a plurality of channels having different wavelengths are multiplexed into as a composite optical signal that is transmitted across a transmission medium. The multiplexed optical signal is received and demultiplexed into a plurality of signals having different wavelengths, and each of these demultiplexed signals are detected and grouped as separate channels according to their wavelengths. Accordingly, wavelength division multiplexing methods permit the efficient expansion of optical communication capacity, and allows for data to be transmitted regardless of the transmission data type.
A typical wavelength division multiplexed optical communication system includes a central office and at least one remote node. In order to transmit data to subscribers, the central office multiplexes a plurality of downstream channels having different wavelengths into a downstream optical signal so as to output the multiplexed optical signal. Further, the central office detects upstream channels output from the subscribers. The remote node is located between the subscribers and the central office to relay the central office to each subscriber.
In the wavelength division multiplexing, some of the types of light sources that can be used are a plurality of single-wavelength light sources or an incoherent multi-wavelength light source. Distributed feedback lasers or fabry-perot lasers can be used as the single-wavelength light sources, and an erbium doped fiber amplifier or a light emitting diode (LED) can be used as the incoherent multi-wavelength light source.
Each of the single-wavelength light sources generates only one mode-locked channel so as to have a single wavelength by a laser resonance. Therefore, the single-wavelength light sources are advantageous in long-distance transmissions because they are coherent sources. Also, the channel power loss and occurrences of noise are minimized in the single-wavelength light sources.
However, one drawback of using single-wavelength light sources is that a plurality of single-wavelength light sources must be provided to the system in order to correspond respectively to the number of transmitted channels. This one to one correspondence increases not only the size of the wavelength division multiplexed optical communication system increases, but also the manufacturing costs.
Meanwhile, the multi-wavelength light sources, such as the LED, have been proposed as an alternative for solving the above-mentioned problems of the single-wavelength light sources. However, the multi-wavelength light sources have the problem that they output incoherent light. Therefore, in comparison with the single-wavelength light source, the multi-wavelength light sources are disadvantageous in a long-distance transmission.
In order to overcome the disadvantages of both the multi-wavelength light source and the single-wavelength light source, a fabry-perot laser with EDFA (Erbium Doped Fiber Amplifier) has been proposed for use in a method of generating and amplifying multi-wavelength light including channels having different wavelengths.
However, the fabry-perot laser does not solve all the above-mentioned problems, particularly when used in multi-wavelength light source. The multiple channels output from the laser are subject to power fluctuation that commonly occurs, thereby increasing relative intensity noise.